ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 955 



(1) All the parts of the system consist of an internal nervous substance 

 and an external sheath. (2) The latter (the subcuticular fibrous tissue) 

 is a fibrous modification of the subcuticula. (3j The former consists of a 

 cortex of ganglion-cells imbedded in the meshes of the subcuticular fibrous 

 tissue, and of a central substance inclosed by the above and formed from 

 ganglionic processes. 



(4) The ganglion cells are all unipolar and raembraneless. They are 

 either (a) small, clear, pyriform, disposed in packets, and with numerous 

 equal-sized nucleoli, or (6) large, darkly granular, round isolated cells, 

 with one large refractive nucleus, or sometimes two differing in character. 

 They are sometimes of enormous size. The two types are connected by 

 transition forms. (5) The cell consists of two substances (a) a granular, 

 fibrillar mitom, and [b) an apparently homogeneous intermediate substance, 

 the j)aramitom. 



(6) The central substance consists of fine, non-anastomosing fibrillae, 

 regularly disposed across one another in the brain, but longitudinal else- 

 where. The central substance is but sparsely penetrated by processes of 

 the subcuticular sheath. In the nerves the fibrils do not form fibres. 



(7) The delicate processes of the first type of ganglion-cell pass 

 directly, those of the others by brush-like terminations, into the fine fibrils 

 of the central nervous substance. The length of the processes before they 

 fall into fibrillse is very variable, it depends partly on the size of the cell. 

 Most of them break up in the same segment, while others are extremely 

 long (giant fibres), extending along the entire system, and even following 

 the nerves to the periphery. (8) The giant nerve-fibres consist of an axial 

 cylinder (the process of the giant-cell), and a fibrous sheath. In those of 

 the ventral nerve-cord, there is a wide space between sheath and axis ; this 

 is traversed by fine lateral fibrils penetrating the fibrous sheath, and 

 possibly connecting the axial cylinder with the fibrils of the central 

 substance. (10) Both the giant fibres and the central substance include 

 very peculiar, small, round elements, like small multipolar ganglion-cells. 

 They give off 3-4 fine fibrils, which, in the central substance, mix with 

 the ordinary nerve-fibrils, and in the giant-fibres pass across the space like 

 the fibrils above noted as arising from the axis-cylinder. This arrange- 

 ment probably secures a second connection between the axis-cylinder of 

 the giant-fibres jind the central nervous substance. (H) The processes of 

 the peripheral ganglion-cells stand in the same relation to the nerves, as 

 the processes of the central ganglion-cells to the brain and nerve-cord. 



Formation of Germinal Layers in Dasychone lucuUana.* — M. L. Eoule 

 has endeavoured to settle the question as to the origin of the mesoblast in 

 polychsBtous annelids, which has been differently answered by Dr. Hatschek 

 and Prof. Salensky, He finds that the ova of Dasychone, which are richly 

 supplied with yolk, segment very irregularly ; of the first two segmentation- 

 spheres, one is small and contains the greater part of the germinal material, 

 and the other is larger and is formed of a compact mass of vitelline granu- 

 lations. The former divides more rapidly than the latter, and its segments 

 gradually surround the yolk until only one point is left uncovered. This 

 corresponds to the blastopore of the larvaB of Eupomatus studied by 

 Hatschek ; at this stage a cavity appears in the region opposite to the 

 blastopore. From the inner layer, at the time when the blastopore closes, 

 some cells separate which will give rise to the mesoblast; in all the 

 sections which the author examined, the number of initial mesoblast cells 

 appeared to be more than two. The central mass of elements charged 



* Comptes Kendus, cv. (1887) pp. 236-7. 



